Slurry pumps are often configured as centrifugal pumps, which employ centrifugal force to lift liquids from a lower to a higher level or to produce a pressure. Basically, a slurry pump comprises an impeller consisting of a connecting hub and shrouds with a number of vanes rotating in a volute collector or casing. Liquid is led into the center of the impeller and is picked up by the vanes and accelerated to a high velocity by the rotation of the impeller and discharged by centrifugal force into the casing and out the discharge. When liquid is forced away from the center, a vacuum is created and more liquid flows in. Consequently there is a flow through the pump.
Centrifugal pumps may be configured as single stage, single suction pumps having an impeller connected to a shaft and sandwiched between a front and back shroud. The rotation of the impeller vanes results in a higher pressure in the volute collector or shell than in the suction, which results in a flow. The higher pressure zone of the volute collector is sealed against the low pressure zone of the suction where the shaft (at a lower atmospheric pressure) enters the collector to avoid leakage losses and loss of performance. On the front or suction side, the most common method of sealing is to use a close radial clearance between the impeller and the casing.
The solids/liquid mixture moved through the slurry pump induces great wear and shortens the pump's life. Wear occurs mostly as a result of particles impacting on the wetted surfaces. The amount of wear depends on the particle size, shape, specific gravity of the solids hardness and sharpness most of which is dictated by the service and the velocity of the impacts and the number (or concentration) of impacts.
In the front sealing gap area, there is relatively high velocity between the stationary liner surfaces and the rotating impeller surfaces and a restricted area, which increases those relative velocities and the number of particles in a given location. Particles being thrown off a rotating radial surface can cause high wear on any close stationary radial surface and that it is better to have an axial (or semi axial) sealing gap.
Various methods have been devised to reduce the wear on the nose gap area. For example, to decrease wear some designs employ a water flush as shown, while others utilize semi axial gaps tapering inwards at some angle to the vertical and still others utilize front clearing vanes protruding out of the front shroud of the impeller into the gap between the impeller and the suction liner.
The front clearing vanes develop a pressure similar to the impeller vanes. The clearing vanes pump the leakage flow from the collector to the suction, thereby reducing wear in the nose gap area. However, it is difficult to maintain a close clearance between the suction liner and the clearing vanes, allowing a gap that particles can use to travel down the surface of the suction liner and through the nose gap. Depending on the clearances, there is a small flow recirculating in the gap between the shrouds and the suction liner and depending on the size of the clearing vanes an even smaller flow across the nose gap.
In spite of using wear resistant materials and various methods for reducing wear, there remains a need for reducing the wear in the high wear areas of a centrifugal slurry pump.